Add a shadow to the navbar

Adapt the bulma-shadow css variable to provide an adequate shadow for the navbar.

sass/_menu.scss

@@ -54,6 +54,9 @@
 
 #main-navbar {
     background-color: teckids.$orange;
+    // adapted from the --bulma-shadow css variable
+    box-shadow: 0 .5em 1em -.125em hsla(var(--bulma-shadow-h), var(--bulma-shadow-s), var(--bulma-shadow-l), 0.3),
+        0 0 0 1px hsla(var(--bulma-shadow-h), var(--bulma-shadow-s), var(--bulma-shadow-l), 0.05);
 }
 
 #main-navbar .navbar-brand > a {